Dept. of Electrical Engineering University of Colorado Denver Denver, CO

Email: jaedo.park@ucdenver.edu

Abstract—A simple flywheel energy storage using a squirrel- cage induction machine is proposed in this paper. The suggested motor/generator system operates with an open-loop Volt/Hertz control scheme and utilizes only the nameplate data as machine parameters. Therefore complex controller tuning or machine parameter measurement is not required. Also, any communi- cation between storage units or with other controllers is not necessary because the system uses bus voltage information for charge/discharge operations. The proposed system has an advan- tage on parallel operation because adding/replacing of units are straightforward. Hence it can easily operate with different types of storage or distributed energy sources in DC bus microgrid systems. Moreover, the proposed control scheme improves the overall stability of the DC bus system. The proposed system has been validated with Matlab simulation and an experimental setup is under construction for verification.

I. INTRODUCTION

Distributed generation systems have recently been inten- sively researched and developed, especially in conjunction with renewable energy sources such as wind turbines and photovoltaic systems. The advantages of distributed generation systems include the capacity relief of transmission and distri- bution, better operational and economical efficiency including effective use of waste heat, and improvement of reliability, eco-friendliness and power quality [1]–[3]. Considering the current energy situation and the energy policy of governments worldwide, the penetration of distributed generation using the renewable energy sources is expected to increase rapidly.

As a way to realize the distributed generation system, a ”microgrid” system that combines distributed energy sources and loads as a small-scale power system can be used. The microgrid approach reduces or eliminates central dispatch and enhances the power quality to sensitive loads [4], [5]. A microgrid is controlled and operated quite differently than a conventional power systems: the distributed energy sources are connected through power electronics converters and the islanded operation is required as well as grid-connected oper- ation [6].

Various kinds of dispachable and non-dispachable prime movers can be included in a microgrid system, such as diesel generators, microturbines, fuel cells, wind turbines and photo- voltaics, to support different types of loads. Distributed energy

Fig. 1.

Conceptual diagram of a DC-bus microgrid system.

storage systems including batteries, flywheels and superca- pacitors are also included. The energy storage capability is important for microgrid operations because the stored energy can be utilized for different purposes such as load support, frequency control, power compensation, and voltage leveling. This is especially true for systems using a high portion of sustainable energy sources; for example, for a microgrid system that is operating in a remote area isolated from a power grid, storage devices play a critical role because of the intermittent and uncontrollable nature of the renewable energy sources.

A flywheel energy storage system using a squirrel-cage induction machine is proposed in this paper. The proposed system utilizes the squirrel-cage induction machine, which is widely available and inexpensive, and the simple Volt/Hertz control technique with just nameplate data as machine param- eters. Therefore no complex parameter measurement is nec- essary and the system has an advantage on parallel operation because adding/replacing units are straightforward. Hence it